Add series module for time-series data

Introduce the `series` module to handle time-series data storage and
manipulation. This module includes:

- `RingBuffer`: An efficient ring buffer for storing time-series data
  with a configurable lookback.
- `ScalarSeries`: A type-safe series for primitive scalar values (f64,
  i64, bool) optimized for performance.
- `ValueSeries`: A fallback series for non-scalar or mixed data types.
- `RecordSeries`: A "Struct of Arrays" implementation for records,
  splitting fields into parallel series for efficient access.

src/ast/rtl/mod.rs

@@ -2,6 +2,7 @@ pub mod core;
 pub mod datetime;
 pub mod intrinsics;
 pub mod math;
+pub mod series;
 pub mod type_registry;
 
 use crate::ast::environment::Environment;
@@ -10,4 +11,4 @@ pub fn register(env: &Environment) {
     core::register(env);
     datetime::register(env);
     math::register(env);
-}
+}